Entanglement of atomic spins beyond simple spin squeezing
Instituto de Ciencias Fotónicas
I describe our recent work in generating exotic entangled states a sample of laser-cooled atomic spins. We use quantum non-demolition (QND) measurement techniques  to prepare entangled spin states. In the simplest case, we measure a single spin component and generate spin-squeezing in a sample of highly polarised atoms - a useful resource for quantum-enhanced atomic magnetometry . More recently, we have developed techniques for squeezing all three spin components of an unpolarised sample of atoms. This generates a highly entangled macroscopic spin singlet (MSS) [3,4], analogous to the ground state of many fundamental spin models in condensed matter physics. The state we generate is SU(2) invariant, so it is directly useful for background-free measurement of magnetic field gradients, and may be useful for quantum information tasks such as storing information in a decoherence free subspace. Combining this with quantum feedback control  should allow us to deterministically prepare a MSS. In the outlook I discuss prospects for using these techniques for quantum state engineering of quantum lattice gases .
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